Building Acoustics

Sound transmission 251

addition we have also plotted the resonant part separately. As evident from the figure, the
resonant part is nearly negligible below the critical frequency, whereas it makes up the
dominant contribution around and above this frequency.

Figure 6.25 Sound reduction index of 6 mm glazing. Calculated result for resonant and total (resonant plus non-
resonant) transmission. Measured data reused by permission from Quirt (1982).

6.5.3 Sound transmission for inhomogeneous materials. Orthotropic panels

As distinct from e.g. glass, a number of other building construction elements cannot
reasonably be characterized as homogeneous and isotropic. The latter term means that
the element is composed of a material whose properties are independent of direction. The
material properties of commonly used panels, however, may vary in the transverse
direction as well as laterally. A typical example of the former type is sandwich elements,
commonly an assembly of three layers but there may also be more layers. We shall defer
the treatment of such elements to Chapter 8, which concerns sound transmission through
partitions having several layers, double walls etc.
In this section we are concerned with orthotropic plates, a commonly used term for
plates where the elastic properties are different in two axial directions. For plane plates
this is caused by material anisotropy, this is typical for wooden materials where the
bending stiffness depends on the direction of the fibres. Fibre-reinforced materials are
another example. Another large class of building elements, which are orthotropic, are
corrugated plates. Plates having a “wavy” corrugation are well known but more common

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Frequency (Hz)

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Sound

reduction index (dB)

Measured

R - total

R - resonant